The present invention relates to electrical compensation for fiber dispersion and laser-induced distortion in analog optical systems.
Analog video signals are often transmitted via Hybrid Fiber Coax (HFC) networks using Dense Wavelength Division Multiplexing (DWDM), in which each channel is amplitude modulated onto a separate subcarrier with the combined RF signal modulating the laser source. The subcarriers are narrowly spaced, for example by 6 MHz each in the NTSC channel plan. (As used herein, a “subcarrier” is a type of “carrier”, so that either term may be used herein to refer to the subcarrier.)
Direct modulated DFB lasers (DML) have been widely used in HFC networks. In forward application 1310 nm single wavelength DML is the predominate technology due to its ability to carry the full bandwidth of signals to meet required system performance. Recently, channel loading for HFC networks has expanded from 50-870 MHz to 50-1000 MHz. 1550 nm DML on the other hand, is used for DWDM (Dense Wavelength Division Multiplexing) in conjunction with 1550 nm externally modulated transmitters for narrowcasting applications. Direct modulated optical laser sources introduce a modulation-dependent frequency deviation of the laser output known as laser chirp. Coupled with fiber dispersion, chirp can produce unwanted artifacts that degrade system performance. For both 1310 nm DML and 1550 nm externally modulated transmitters, the dispersion has not been a problem due to the absence of dispersion in optical fiber at 1310 nm and absence of chirp in externally modulated transmitters. 1550 nm DML transmitter for narrowcasting on the other hand does have a chirp induced dispersion problem, but it is not so severe as to substantially degrade system performance because the number of channel transmitted is very small (between 50 and 300 MHz).
With a changing business environment that now demands both wide bandwidth and low cost for expand HFC networks, and with advances in DFB laser technology, DML based transmitters are becoming a better choice than externally modulated transmitter for DWDM applications due to their significantly lower cost and simplicity. It is therefore becoming essential to overcome dispersion degradation for the technology to work well enough to meet system requirements. This is true for both 1310 nm and 1550 nm DWDM transmitters.
It is well known to mitigate the effects of both laser chirp and chromatic distortion by precompensating the RF modulation signal before it is applied to the transmitter. The basic concept is that a set of distortion signals are produced in advance by a distortion generator circuit, which are equal in magnitude but opposite in phase to the characteristics of the nonlinearity to be compensated. When these predistortion signals interact with distortion generated by the system nonlinearities they cancel each other out, thereby reducing or removing the distortion that would otherwise be generated.
The distortion caused by both laser chirp and dispersion in the fiber is so-called second order distortion. Laser chirp distortion has a frequency independent term and a frequency dependent term, whereas the distortion introduced by dispersion in the fiber has only a frequency independent term, the frequency independent term being negligible. The frequency dependent term caused by laser chirp distortion also includes a 90° phase shift. Another source of distortion, that introduced by the fiber amplifier (if present), also has only a frequency independent term. A multi-path predistortion scheme for analog optical transmission distortion compensation based on these observations is described in Kuo et al. in “Second-Order Dispersion and Electronic Compensation In Analog Links Containing Fiber Amplifiers,” Journal of Lightwave Technology, Vol. 10, No. 11, pp. 1751-1759 (1992), incorporated by reference herein. Kuo's scheme collects all the frequency independent terms separately from the frequency dependent terms, precompensates them separately, and recombines them with the original RF signal to yield a signal that precompensates for all three sources of distortion.
FIG. 1 is a block diagram illustrating this scheme. As shown in FIG. 1, the scheme involves branching the input RF signal into three parallel paths and then recombining them for delivery to the laser driver. One path 112 introduces frequency dependent precompensation, and a second path 114 introduces frequency independent precompensation. The third path 116 carries the original signal, delayed to match the delay in the paths 112 and 114.
Referring to FIG. 1, the input RF signal is provided to a splitter 110 having three outputs defining the three respective parallel paths. In the frequency dependent compensation path 112, the RF signal is first squared in squarer 118. The squared signal is then passed through a variable attenuator 120 and then an amplifier 122, and then differentiated in a differentiator 124. The differentiator is provided to effect both the frequency dependence and the 90° phase shift. In the frequency independent compensation path 114, the RF signal is squared in squarer 126, then attenuated in variable attenuator 128. No frequency dependence or phase shift is included. In the third path 116, the RF signal is merely delayed in physical delay element 130 in order to match the delay in the other two paths. The outputs of the three paths are recombined in combiner 132 for delivery to the laser source. Similar schemes are disclosed in Pidgeon U.S. Pat. No. 5,481,389 and Gottwald U.S. Pat. No. 5,526,159, both incorporated by reference herein.
Unfortunately, all three proposals have severe limitations for broadband applications that carry the full channel loading from 50 to 1000 MHz. Precompensation using the known schemes for laser chirp and fiber dispersion typically will not meet tight specifications for full wideband channel loading using direct modulated laser sources operating either at 1310 nm or 1550 nm.
Higher performance allows the system to reach longer distances. Therefore, to make high performance DWDM DML transmitters, better distortion cancellation is required and high performance circuitry is required to achieve high performance. The invention described herein addresses these problems.